This invention relates generally to an improved outlet structure for a liquid processing vessel of a media mill, often referred to as a sand mill. In particular, the outlet structure includes an improved screen assembly and the related construction for mounting the screen assembly with respect to the sand mill vessel.
Sand milling is a proven, practical, continuous, high production method of dispersing and milling particles in liquid to produce smooth, uniform, finely dispersed products. Some of the products for which the sand milling process is used include paints, inks, dye stuffs, paper coatings, chemicals, magnetic tape coatings, insecticides, and other materials in which milling to a high degree of fineness is required.
In a typical sand milling process, the material or slurry to be processed is introduced at one end of the processing chamber or vessel and pumped through a small diameter grinding media while a rotor within the vessel agitates the media to insure proper milling and dispersion of small particles in the liquid or slurry being processed. Although the grinding media in years past was sand, more currently a small manufactured product of steel, glass or other material is used.
The processed liquid exits from the vessel, but the grinding media must, of course, remain within the vessel. To accomplish this, the outlet structure typically includes a screen assembly which prevents the media from leaving the vessel while the processed liquid flows through the screen. U.S. Pat. No. 4,441,658 issued Apr. 10, 1984 describes a cup-shaped assembly that fits within a cylindrical wall leading to an outlet. The cup shape of the screen assembly provides a large filtering surface area. Other screen assemblies include segments forming a portion of a cylindrical wall. These screen components are typically formed of small diameter rods or strands which are welded at their intersections. A shortcoming of these welded constructions is that the screen becomes worn, causing some of the strands of the screen to break or causing the openings between the strands to become large enough to allow passage of the grinding media. This requires early replacement of the screen.
It is also desirable that the screen assembly extend into the vessel so that the entire surface area of the screen is directly exposed to the flow of liquid through the vessel, and so that the grinding media cannot become lodged between the screen and closely surrounding walls, thus obstructing a portion of the screen. In Wilhelm (U.S. Pat. No. 3,780,957) a screen assembly for a media mill is shown in which a plurality of tubular screen cartridges extend into the internal space of the media mill vessel. However, since the screens extend into the vessel they are subject to even greater wear by the abrasive grinding media. The system in Wilhelm is unsatisfactory since the screening wears too quickly.
Thus, a need exists for an improved longer lasting screen construction for a media mill. The screen vessel, together with the outlet structure must also be arranged so as to provide easy disassembly for cleaning or replacement of the screen components.